1. Field of the Invention
The present invention comprises a method for labeling an identification mark on a plastic spectacle lens, and more particularly, the present invention relates to a method of using an ink jet printer or a similar printing device to print an identification mark onto a mold used to form a lens and subsequently transferring the identification mark from the mold directly to the lens when the lens is formed.
2. Background Art
The ophthalmic lenses for glasses are made of a transparent material, usually glass or plastic, and are of a size and shape to produce desired effects, namely, focusing the light for the person""s eye to see clearly. Such glasses or spectacles must correspond to a person""s prescription as well as to the person""s morphological and psychological characteristics. In other words, lenses used in glasses have certain optical properties corresponding to a set of specifications as described, say, in a prescription.
The lenses use a well-defined geometrical configuration which determines the their optical properties. The shape of each lens is characterized by three attributes: (1) the curvature of its two surfaces; (2) the thickness at its center and edges; and (3) its diameter. The two surfaces of a lens can use various geometric configurations, including the following shapes: spherical; cylindrical; toric; plano; aspheric (usually elliptical); and progressive. For example, the surface of a lens can have a constant radius along its different axes so that the surface is symmetrical, which is known as a spherical surface. The spherical lens surface mirrors the shape of a portion of a sphere in which all meridians have the same radius of curvature. The spherical surface may be either convex or concave.
Alternatively, the surface of the lens can have two axes, each having a different radius of curvature, so that the surface of the lens is asymmetrical. An astigmatic surface is an example of such an asymmetrical surface and is characterized by its two principal meridians having a different radius of curvature from each other. The meridian having the greatest radius of curvature is called the xe2x80x9caxis,xe2x80x9d and the other meridian having the smaller radius is called the xe2x80x9cperpendicular axis.xe2x80x9d Astigmatic lens surfaces predominantly include a cylindrical surface and a toric surface. A plano surface and aspheric surface are examples of other lens surfaces used in the art.
For the cylindrical surface, the principal meridians along the axis have an infinite radius of curvature, e.g., flat or straight, and the perpendicular axis has a radius of curvature which is the same as the circular radius of a cylinder. Thus, a concave cylindrical surface is shaped to complementarily receive a cylinder on the surface and a convex surface resembles the exterior surface of such a cylinder.
The toric surface resembles the lateral surface of a torus, e.g., shaped as the inner tube of a tire. Thus, a torus surface is similar to a cylindrical surface, but the longitudinal axis curves instead of being straight as for a cylindrical surface. The perpendicular axis or meridian on the toric surface has a radius of curvature smaller than the radius of the axis. As with a spherical and a cylindrical surface, a toric surfaces can be convex by having the shape of the exterior surface of a torus or, alternatively, may be concave by having the shape of the inner surface of a torus.
An astigmatic surface is used for a person with an ocular astigmatism, in which the cornea is elliptical instead of round. The orientation of the elongated portion of an astigmatic cornea varies from person to person. For example, one person may have an axis at five degrees, another at thirty degrees, and another at yet a different orientation. The axis of the surface of the lens must be oriented to align with the orientation of the elongated portion of the cornea.
Different lens surfaces can be used in combination. Often, the front surface of a lens is spherical and the back surface is spherical, cylindrical, or toric. The front surface can alternatively be a plano surface. The optimum combination of surfaces in a lens is determined by the optical properties, the proposed use, and the appearance of the lens.
In addition to shape, thickness is also an important characteristic of a lens. The glass or plastic used to form the lens is a factor in establishing the thickness. Many lenses today are made from plastic because of its light weight, density, refractive index, and impact resistance. Examples of plastics used for lenses include methylmethacrylate (a thermoplastic resin, which is better known by its trademark xe2x80x9cPlexiglasxe2x80x9d(copyright) or xe2x80x9cPerspexxe2x80x9d(copyright)) and diallyl glycol carbonate, which is also known as CR39.
CR39 is one of popular lens-forming materials used today, in part, because all lens types used in ophthalmic optics can be made from it by molding. CR424 is another lens-forming material used today. CR39 is a petroleum derivative of the polyester group, a family of polymerisable thermosetting resins. In production, a monomer is first obtained from CR39. The monomer, which is a limpid liquid with the viscosity of glycerine oil, remains in a liquid state in cold storage, but hardens after several months at room temperature. To form a lens, the liquid monomer is placed and contained in a cavity or volume jointly defined by two molds and a closure member such as a gasket. Once the monomer is in the volume, the monomer is cured to form a hardened polymeric lens taking the shape of the molds.
The glass molds used to form polymeric lenses are important in lens manufacturing by molding. Not only do the molds form the correct shape to the lens according to the optical characteristics required, but the surface qualities of the finished lens depends on the accuracy of the molds since the lens surfaces are a precise reproduction of the inner mold surfaces. Accordingly, the mold surfaces are prepared with extreme precision and, after manufacture, are heat toughened to withstand the strain of the polymerization process.
The relative axial positions of the molds are also important in lens molding because they decide the thickness of a lens. As people skilled in the art know, different relative axial positions of the molds producing lenses with varying powers. The molds would be set farther apart to form a lens of a greater power compared to form a lower power lens. Thus, for a specific power lens, molds must be set at a predetermined axial separation.
An add power front mold, which forms a bifocal or trifocal portion to the lens, can also be used in forming lenses. The add power mold includes a segment curve, which is a concave depression cut into the concave half of the mold, to form the add power segment on the front surface of the lens. This segment curve produces a convex surface for the distance portion, together with a steeper convex surface for the reading add power segment.
In sum, each optical lens has a unique set of specifications identifying its optical properties. Because a lens formed by molding takes the shape of the molds, the specifications of the lens are determined by the corresponding specifications of the molds and the relative positions of the molds.
While some lenses are still made in a traditional way, most lenses today are made by molding for good reasons. One is that molding produces better lenses because the molds can be prepared with great precision and plastic such as CR39 or CR424 conforms to the shape of the molds easily. Moreover, molds are reusable and therefore molding reduces the production cost. Furthermore, molding allows the plastic lens forming process to be easily automated and thus further increases productivity at reduced production cost.
In an automated lens-forming manufacturing process, more than one manufacturing line can be utilized to produce lenses in quantity. Often one manufacturing line may produce lenses with a set of specifications. Other lines may produce lenses with one set of specifications. Because each lens has its own set of specifications, it must be properly labeled with corresponding specifications before it is delivered to customers.
However, tracking the specifications of an individual plastic spectacle lens after its manufacture and verifying its identity is troublesome because lenses are transparent. They look alike even if they are not the same. Currently, an additional inspection step must be used in the lens manufacturing process to measure and identify a lens once it is produced. Moreover, depending on when the lens is labeled on the manufacturing line, mix-ups may occur. Because lenses made at a production site are manufactured from the same plastic, and often from the same molds, the likelihood of mix-ups is even greater today. To minimize mix-ups, it may be desirable, or even necessary, to verify the lens identity before labeling. Thus, additional time and effort have to be spent for proper inventory control. This prolongs the manufacturing time and increases the production cost. The problem has existed for years but it is still left unaddressed.
The present invention attempts to solve the inventory control problem of molded lenses by providing a method for labeling an identification mark on a molded plastic optical lens. The identification mark includes information identifying the lens"" optical properties including at least the power of the lens. The identification mark may be visible and readable to human eyes. Preferably, the identification mark is machine readable. The lens is labeled with the identification mark by applying the identification mark in ink to one of the molds used to form the lens. The identification mark is transferred from the mold to the lens upon curing and removal from the mold.
In this regard, the method of the present invention in one embodiment is practiced by placing an identification mark in ink on the facing inside surface of a mold. The mold is then filled with a lens-forming liquid. The lens-forming liquid is cured or hardened to form the lens while the identification mark is remained on the mold. The identification mark is transferred from the mold to a portion of the surfaces of the formed lens by virtue of direct contact between the mold and the surfaces of the lens because the ink has stronger affinity to the lens material than to the mold. When the lens is removed from the mold, the identification mark stays with that portion of the surfaces of the lens and can be used to identify the lens for inventory control purpose.
Thus, the identification mark of the lens is given to and labeled on the lens when it is produced, or xe2x80x9cborn.xe2x80x9d Consequently, the present invention may minimize the likelihood of potential mix-ups. Moreover, the additional inspection procedure is no longer needed to identify the characteristics of the lens. A reading of the identification mark can provide information identifying the lens"" properties including the power of the lens.
Because a molded lens is formed normally by coordinating a first mold and a second mold whose facing inside surfaces are a negative image of the surfaces of the lens when the molds are positioned at a proper distance and rotational orientation to each other, either mold can be chosen as a candidate to receive the identification mark. The present invention can be practiced by placing the identification mark on the facing inside surface of the first mold or the second mold or both. In one embodiment, the first mold is chosen to receive the identification mark. The first and second molds, cooperating with a closure member to form a molding cavity, are then filled with a lens-forming liquid such as a CR424 monomer. The lens-forming liquid is cured or hardened to form the lens while the identification mark remains on the first mold. The identification mark is then transferred from the first mold to a portion of the surfaces of the lens and remains with that portion of the surfaces of the lens when the lens is removed from the molds. The first mold can be a front mold or a back mold, as known to the people skilled in the art.
In one embodiment, the present invention uses an ink jet printer to place the identification mark onto the facing inside surface of a mold. The ink composition is selected to allow the identification mark to remain on the mold when the lens-forming liquid is filled and then cured. If the mold is made from glass and the lens-forming material is polymer, the ink is selected to have stronger affinity to the polymer lensforming material than to the glass mold. Once the lens is formed, but before demolding, the identification mark is transferred directly from the mold to a portion of the surfaces of the lens. The transferring is achieved because the ink has stronger affinity to the lens. Thus, the transferring is accomplished automatically. The identification mark contains information identifying the lens"" properties such as the power of the lens. Thus, the specifications of the lens can be easily verified from the identification mark on the lens. Chances for error in labeling can be minimized because the identification mark is transferred to the lens directly from the mold that determines the lens"" properties.
Because the specifications of a molded lens depend on the physical properties of molds from which the lens is formed, each mold can be premeasured and marked with an indication such as a mark in the form of a bar code that represents the measurements. This bar code can be etched onto the back surface of the mold. The back surface is the surface opposing the facing inside surface of the mold where the lens-forming liquid is to occupy in filling. In use, an operator, being a real person or an automation means, can just scan the bar code to get the properties of the mold without repeating the measurements. The bar code can be read by a machine such as a scanner and then placed in ink on the facing inside surface of the mold so that the bar code transfers subsequently onto the cured lens. The bar code on the back surface of the mold could then be read again and reprinted on the facing inside surface of the mold before the mold is used to make another lens. This process can be repeated and thus is suitable for use in automated lens-forming manufacturing process.
When the identification mark is printed onto the mold, the location of the identification mark on the facing inside surface of the mold may be anywhere within that surface. But preferably, the identification mark is located at a spot so that when the identification mark is transferred to the lens it will be positioned at a location that will be cut away when the lens is finally used in glasses. Normally, that location is around the periphery of the facing inside surface. Alternatively, the identification mark can be removed from the lens by polishing near or at the end of the manufacturing process.
Furthermore, the present invention utilizes a computer system to coordinate the labeling process and thus has the capacity to put an identification mark on the mold with updated information. As known to the people skilled in the art, normally two molds are required to form a lens. Thus, even if a mold is measured beforehand and its physical properties such as its height, radius and other characteristics are ascertained and contained in the identification mark, such as a bar code, the thickness of the lens to be formed can only be ascertained when the two molds are positioned at a proper axial separation. Once the molds are positioned properly, the computer system would obtain the corresponding data for the thickness of the lens and group it together with the information already contained in the bar code into an updated identification mark. The updated identification mark then is placed onto the facing inside surface of the mold. The updated identification mark can also be in a form of bar code.
Other objects, advantages and uses for the present invention will be more clearly understood by reference to the remainder of this document.